Category Archives: Raspberry Pi

Post navigation

This blog originally started when I took delivery of the DyaBot, a Raspberry Pi and Arduino based variant of the C3Pi running Dyalog v13.2. The architecture of the ‘bot and instructions for building your own inexpensive robot can all be found in blog entries from April to July of last year.

The downside of only using inexpensive components is that some of them are not very precise. The worst problem we face is that the amount of wheel movement generated by the application of a particular power level varies from one motor to the next, and indeed from moment to moment. Trying to drive a specific distance in a straight line, or make an exact 90 degree turn regardless of the surface that the ‘bot is standing on, are impossible tasks with the original Dyabot. You can have a lot of fun, as I hope the early posts demonstrate, but we have higher ambitions!

Our next task is to add motion sensors to the DyaBot, with a goal of being able to measure actual motion with sufficient accuracy to maintain our heading while driving straight ahead – and to make exact turns to new headings, like the 90-degree turn made in this video (the ‘bot has been placed on a Persian carpet to provide a background containing right angles):

Introducing the MPU-6050

For some time we have had an MPU-6050, which has 3-axis rotation and acceleration sensors, attached to our I2C bus, but haven’t been able to use it. About ten days ago (sorry, I took a few days off last week), @RomillyC came to visit us in Bramley to help me read some Python code that was written for this device. The current acceleration or rotation on each axis is constantly available as a register and can be queried via I2C. Translated into APL, the code to read the current rate of rotation around the vertical axis is:

Integrating to Track Attitude

Reading the register directly gives us the instantaneous rate of rotation. If we want to track the attitude, we need to integrate the rates over time. The next layer of functions allows us to reset the attitude and perform very primitive integration by multiplying the current rotation with the elapsed time since the last measurement:

We can now write a function to rotate through a given number of degrees (assuming that the gyro has just been reset): we set the wheels in motion, monitor the integrated angle until we are nearly there, then shut off the engines. We continue logging the angles for a brief moment to monitor the deceleration. The function returns a two-column matrix containing the “log” of rotation angles and timestamps:

The logged data from the rotation captured in the video at the top is charted below:

Even with the primitive integration and rotation strategies, the results already look quite promising. I’ll be taking most of this week off – part of it without access to the internet(!), but once I am back, expect the next blog entry to explore writing functions that accelerate and slow down in a more controlled fashion as well as stop at the right spot rather than relying on a specific amount of friction to rotate through the last 7 degrees (note the very slight reverse rotation at the end, probably caused by the Persian carpet being a bit “springy”). I will also clean up the code and post the complete solution on GitHub – and perhaps even look at some better integration techniques.

Last week the ‘bots were busy preparing for the J Language Conference in Toronto, where they made their first public appearance together. Upon returning to Bramley they continued training and we are proud to present the first recording of their new dance:

The ‘bots are both running the same DyaBot class as last year. This class exposes a property called Speed, which is a 2-element vector representing the speed of the right and left wheels respectively. Valid values range from +100 (full speed ahead) to -100 (full reverse). The annotations displayed at the top left show the settings used for each step of the dance.

Controlling Two Robots at Once using Isolates

Isolates are a new feature included with Dyalog version 14.0, designed to make it easy to perform distributed processing. In addition to making it easy to used all the cores on your own laptop or workstation, isolates make it possible to harness the power of other machines. This requires the launching of an “isolate server” on each machine that wants to offer its services:

Starting an isolate server on DyaBot00 using PuTTY.

Once we have an isolate server running on each robot we can take control of them from a remote session as follows:

Next, we define a function “run” that will take a vector of dance steps as input. Each step is a character vector (because that makes editing slightly easier!) containing five numbers: The first two set the speed of one robot, the next two the speed of the other and the fifth defines the duration of the step. After each step we pause for a second, to give humans time to appreciate the spectacle:

Bot 04 and Bot 00 hanging out in Rochester NY rehearsing for the J Conference and considering whether to return to Toronto with me for the IPSA 50th reunion on October 4th this year.

The next major step in the robot project is to make use of the tiny red board attached to Bot 00 (on the right) – an MPU-6050 accelerometer. At the Dyalog Seminar in New York last Thursday I finally had the pleasure of meeting @alexcweiner in person, and we vowed to crack this nut together; since @romillyc has promised to join in as well, failure is not an option. Stay tuned to hear more about that adventure in the weeks to come!

Welcome to The Development Team Blog

The really good news is that this blog is no longer simply “the CTO blog” but a blog that will be shared by the entire development team as well as invited guests. We look forward to sharing details of the things we are working on with you all…

Regular readers will remember my whining about the poor precision of both infra-red and ultrasonic sensors. But today, the Raspberry Pi / Dyalog APL – controlled “DyaBot” was observed driving on a dinner table – where the slightest navigational error could mean a 3-foot plunge and certain death! How can this be?

I no longer need to complain about sonar beams being up to 30 degrees wide: the PiCam has a resolution of up to 1080×1920 pixels! So all we need is some software to interpret the bits…First, we capture images using the “raspistill” command:

raspistill -rot 90 -h 60 -w 80 -t 0 -e bmp -o ahead.bmp

The parameters rotate the image 90 degrees (the camera is mounted “sideways”), set the size to 60 x 80 pixels (don’t need more for navigation), take the picture immediately, and store the output in a file called “ahead.bmp”. (Documentation for the camera and related commands can be found on the Raspberry Pi web page.) Despite the small number of pixels, the command takes a full second to execute – anyone who knows a way to speed up the process of taking a picture, please let me know!

In the video, each move takes about 3 seconds, this is simply because each cycle is triggered by a browser refresh of the page after 3 seconds. Capturing the image takes about 1 second, the “image analysis” about 40 milliseconds, so we could be driving a lot faster with a bit of Javascript on the client side (watch this space).

Extracting BitMap Data

Under Windows, Dyalog APL has a built-in object for reading BitMap files, but at the moment, the Linux version does not have an equivalent. Fortunately, extracting the data using APL is not very hard (after you finish reading about BMP files on WikiPedia):

The above gives us a 60 by 80 by 3 matrix containing (R,G,B) triplets. This code assumes that BMP file is in the 24-bit format created by raspistill; I will extend the code to handle all BMP formats (2, 16 and 256 colours) and post it when complete – but the above will do for now.

Where’s the Edge?

At Iverson College last week, I demonstrated the DyaBot driving on a table with a green tablecloth, using code which compared the ratio between R,G,B values to an average of a sample of green pixels from one shot. Alas, I prepared the demo in the morning and, when the audience arrived, there was much more (yellowish) light coming in through the window. This changed the apparent colour so much that the bot decided that the side of the table facing the window was now unsafe, and it cowered in the darkness.

Fortunately, I was talking to a room full of very serious hackers, who sent me off to Wikipedia to learn about “kernels” as a tool for image processing. Armed with a suitable edge detection kernel, I was able to test this new algorithm on a dozen shots taken at different angles with the PiCam. Each pair of images below has the original on the left, and edges coloured red on the right. Notice that, although the colour of the table varies a lot when viewed from different directions – especially when there is background glare – the edges are always correctly identified:

Except for the image at the bottom left, where the bot is so close to the edge that the table is not visible at all (and the edge is the opposite wall of the room, ten feet away), we seem to have a reliable tool.

The PiServer Page

The code to drive the bot is embedded in a “PiServer” (MiServer running on a Rasperry Pi) web page. Each refresh of the page takes a new picture, extracts the bits, and calls the main decision-making function. The suggested action (turn or drive straight ahead) is displayed in the form, and the user has the choice of pressing OK to execute the command, pressing the “Nah” button to take a new photo and try again (after moving the bot, changing the lighting in the room, or editing the code). There are also four “manual” buttons for moving the Bot. After testing the decision-making abilities of the code, brave programmers press the “Auto” button, allowing the robot to drive itself without waiting for confirmation before each command (see the video at the start of this post)!

The Code

The central decision-making function and the kernel computation function are listed below. The full code will be uploaded to the MiServer repository on GitHub, once it is finally adjusted after I find a way to attach the PiCam properly, rather than sticking it to the front of the Bot with a band-aid!

∇ r←StayOnTable rgb;rows;cols;table;sectors;good;size;edges
⍝ Based on input from PiCam, drive at random, staying on table
⍝ Return vector containing degrees to turn and
⍝ #seconds to drive before next analysis
(rows cols)←size←2↑⍴rgb
⍝ First detect edges in each colour separately
edges←EdgeDetectAll∘AK¨⊂[1 2]rgb
⍝ Call it an edge if any r, g or b result is >75% of original
edges←∨/(↑[0.5]edges)>0.75×1 1↓¯1 ¯1↓rgb
⍝ Look for lowest edge in each column
table←+⌿∧⍀~⊖edges
⍝ Divide into 3 equally sized sectors (left, centre, right)
sectors←((⍴table)⍴(⌈(⍴table)÷3)↑1)⊂table
⍝ Find the LOWEST edge in each sector
good←⌊/¨sectors
:If good∧.>15 ⍝ More than 15 pixel rows of table in all sectors
r←0,0.1⌈(good[2]-15)÷25 ⍝ Carry straight on for a while
:Else ⍝ Some sectors have less than 15 pixel rows
:If 0≠1↑previous ⋄ r←previous
⍝ Once started turning, keep turning the same way
:Else
r←((1+>/good[1 3])⊃45 ¯45),0 ⍝ Turn in "best" direction
:EndIf
:EndIf
previous←r ⍝ Remember last turn for next decision
∇

The function AK (Apply Kernel), and the kernel are defined as follows:

Of course, it was an illusion that I would be able to get straight back to the robot after vacation, there were a few other jobs waiting like the Dyalog’13 Conference Programme (the DyaBot will be making a couple of appearances, of course!). However, it appears I have now made enough flour to be able to remove my nose from the grindstone for a bit and spend more time with the robot, so I expect to be back to posting every week or so from now on, as we prepare to demonstrate an autonomously navigating robot at the conference!

Anyhow, here is a chart with the first recorded data from the LV-EZ4 ultrasonic sensor (sonar) - using RainPro, as demonstrated in the post on visualising sensor data. Many thanks to Stefano Lanzavecchia for help with the math required to generate the co-ordinates of a rectangle in a polar chart:

The robot was placed in my kitchen (the setting for several robot videos), about 80cm from the back wall, 240cm from the front, and roughly halfway between the sides. The green rectangle labelled “actual” above shows the location of the walls relative to the bot. It was commanded to rotate through 360 degrees, recording the readings from the IR (red line) and Sonar (blue) sensors, which were both pointing straight ahead. Ideally, the blue line should have traced an identical rectangle. The red line essentially traces an 80cm circle plus noise, this is because SHARP GP2Y0A21YK0F infrared sensor has a maximum range of 80cm.

Sonar Issues

The ultrasonic sensor has a range of at least 6-7m, and is our choice for long-range distance measurements. However, what I believe the graph shows, is that we have a serious problem with reflections. Essentially, it appears that when the sonar beam hits a wall at certain angles, there is very little direct return; instead the beam is reflected. The return signals that we are getting from roughly 60 and 315 degrees look like reflections of the back wall – these reflections may be enhanced by the fact that there is a refrigerator at 60 and a dishwasher at 315 (metal surfaces). The signals at 135 and 220 degrees could be reflections of the robot itself, the beam having been reflected twice in a corner of the room.

Another thing that is not immediately evident from the above image, but took me by surprise studying the data sheet, was that the sonar beam is a quite a bit wider than I expected – as much as 30 degrees at close range, getting narrower as the distance increases (see the data sheet). In other words, we have some work to do in order to generate accurate maps of the universe based on the current set of sensors.

It has been more than a month since my last blog post, and you’d be forgiven for thinking that I’d lost interest in robots. It *is* true that I have been rather distracted by customers, including a trip to the USA. It is also holiday season, and I have been to a music festival. However, except for last week when I was skipper and this week when I am chillng out on the Anna Elise, the robot has been in my suitcase on every trip, and I have actually been working hard on it. I just haven’t been able to get anything new to work until very recently.

One problem was that robots do not travel particularly well. Put them in a suitcase every 3-4 days and bits quickly start falling off (if you have ever looked out the window of your plane and watched a baggage handling crew at work, that should be no surprise). I now have both 220v and 110v soldering irons (Thank You RadioShack!), and have so far been able to put humpty dumpty together again on arrival. But my last desperate attempt to collect some sonar data that I could publish before going sailing ended like this:

Rewriting the Raspberry Pi – Arduino Comms Program

The BIG reason for the lack of visible progress is that I decided that the time had come to rewrite the control program for the Arduino. The robot uses a Raspberry Pi running Dyalog APL as the main “brain”, but because a high-level language on a Pi isn’t a “real time” programming environment, we are using an Arduino to control the I/O pins. The idea is that, by implementing a very simple “front-end processor” on the Arduino, we will have a tool that APL can control at a high level, leaving timing-dependent work like controlling pulse-width-modulated output pins, which need to be continuously updated, to the Arduino.

The old control program had a number of limitations; most importantly, it was only really possible to monitor a single input pin. This was good enough until the sonar was added to the infra-red sensor. The program also contained hard-coded information about how pins were being used. Finally, it was quite simply a badly-written piece of Arduino C, subject to intermittent timing-related glitches because the original authors were unaware of certain limitations of the Arduino.

So I decided to write about 5 pages of new C code, and it took me a month and a bit to get back to blogging. Apart from the distraction of having to pretend to still be the CTO of Dyalog Ltd, the real problem is that I am not (usually) a C programmer. C is a rather unforgiving language. Or rather, it is a VERY forgiving language – it allows you to write code that will compile without warnings but just not work, because you unwittingly asked it to do odd things like truncate the content of a variable that you have used in a context which assumes a shorter type (etc, etc, and etc).

To stack the odds further against me, the default Arduino environment has no real debugger. In addition, the I2C bus that we are using to communicate between the Pi and the Arduino is very timing-dependent, which means that any attempt to step through the code will necessarily cause it to fail. Even the standard Arduino practice of monitoring diagnostic messages written to the serial interface cannot be used as it alters the timing enough to cause many I2C requests to fail. So after 2-3 weeks of wailing and gnashing teeth, I implemented my own simple logging mechanism to return diagnostic info via I2C and finally managed to move very slowly forward and complete the new control program. And I even RTFM and a few web pages written by people who had done this kind of thing before – in particular, this post by Wayne Truchsess was extremely helpful.

I was hoping to be able to publish some results of using the new Sonar, but I tried to do this on a jet-lagged Sunday morning sandwiched between returning from the USA and heading off to sail, and absolutely everything I did went wrong. The batteries died in the middle of my data collection and, to make the day perfect, when I found a new set of batteries, a wheel fell off (see the video above)…so I won’t be able to blog about the sonar data until I am back from the sailing trip at the end of July. However, I would like to talk a bit about the new control program.

I2CArduinoComm v0.2

As previously mentioned, we have a piece or Arduino C code (available on GitHub), which supports a simple command language which APL on the Raspberry Pi uses to issue instructions to set or read I/O pin values. The BIG difference compared to the old version is that ALL code that either reads or sets pin values has been moved to the loop() function, which (if my understanding is correct) is dispatched by the Arduino “operating system” when the O/S is not busy with other tasks. The old program would read and update pins immediately upon receipt of I2C messages. The I2C bus is quite critical with respect to timing, and the old code was spending too much time reading pin values when a data request arrived – so we were missing the timing window for getting a response back in time – and possibly interfering with other timing-dependent activities on the Arduino. The new program constantly maintains arrays containing the latest input values (done in the “loop” function), so all an I2C read request needs to do is to transmit the current values.

Command Language Extensions

From a functional point of view, the main improvement is that the pin usage is completely configurable using a new “setup” command. At a lower level, the command language has been made more flexible. We no longer use a fixed command length; instead the first byte transmitted declares the length of the command (the first byte of all results is also the length of the response). The following commands are supported:

Command
Character

Command Name

Description / Comments

I

Identify

Returns two bytes containing the major and minor version number of the ArdCom program (currently 0 2), followed by two bytes per defined pin (pin #, pin type).

R

Reset

Clears all pin definitions.

S

Setup

Declares pin types. Following the command character, triplets of (pin#, pin type, additional info) for each pin (see the pin type table below for details). “S” can be called several times in succession if the overall length of the command would otherwise exceed 32 bytes).

W

Write

Sets pin values: the command char is followed by pairs of (pin#, value).

Pin Setup

Each pin declaration included in a setup command consists of a triplet of bytes, containing the pin#, type character, and “additional info”. Only the “p” pin type currently makes use of the additional info – for other pin types this value is ignored.

Pin Type
Character

Type Name

Description / Comments

A

Analogue output

Uses the analogWrite function to set the value.

D

Digital output

Uses digitalWrite.

S

Servo output

Uses Servo.write to update the pin value.

a

Analogue input

Uses analogRead.

d

Digital input

Uses digitalRead.

p

Pulse input

Uses pulseIn to read a pulse-width modulated input. If “additional info” is set, then this should be a digital pin number which is given a 10ms HIGH signal to tell the device to provide an input pulse.

See the Arduino Reference for descriptions of analogWrite, digitalWrite and the other functions mentioned above.

Reading Input Values

When an I2C read request is made (by APL on the Pi), the current values for all pins defined as inputs (in ascending order) are returned in a single transmission. The data stream contains two bytes (Most Significant Byte followed by LSB) for each input pin. If the control program wants to return diagnostic information, then the first data byte will have the value 254 and the rest of the transmission will be textual “log” data. 254 is an impossible value for the MSB of any input, because the maximum input value is 1023, which has an MSB value of 3. 254 was chosen rather than 255, which frequently occurs when there are transmission failures or other errors.

The ArdCom Class

A new APL class has been written to communicate with the new Arduino code. You can find the source code in the file ArdCom.dyalog in the GitHub repository. The DyaBot class has been updated to make use of the ArdCom encapsulation, the new version can be found in the Exampes folder in the repository. I will describe the in detail in my next blog post – probably not for another week, as the weather forecast looks superb for the next few days (probable route Faaborg-Aabenraa-Augustenborg-Gråsten-Flensburg-Sønderborg-Ærøskøbing).